i've cold smoked plenny fish (sable/butterfish and salmon), both on the mainland and on island. so it can be done, main thing is you need to keep the fish around 80F max (ideally under 70).
for that reason, you don't need too much if any charcoal at all. just as a way to keep the smoking wood smoldering.
use more brown sugar to season than you think you need.
if you're after the traditional hardwood taste, hickory would be ok. alderwood is good for hot smoke. absolutely no kiawe (sorry couldn't resist).
6 hours was more than enough for fillets.
please look up all the USDA food safety mumbo jumbo before you do this. when cold smoking, the fish can only be held at certain temperature for a certain amount of time before you get da nice kine listeria seasonin'
try freeze the fish after smoking and then slice; definitely helps with the slicing and supposedly helps reduce bacteria (obviously only do this with fillets).